I'm trying since a couple of days to setup a chrooted sftp access to a test obsd machine running apache. Ideally I would like to grant group access to update some web content but after checking all I found, I'm still unsuccessful

The typical problem with a chroot environment is that you need to provide a copy of the necessary configuration into the chroot.

Config files in "/etc" which normally are accessible by a non-chrooted daemon, need to brought into the chroot environment when you run a chrooted version.
In case you haven't seen it, http://www.openbsd.org/faq/faq10.html#httpdchroot has an example.

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I'm experiencing the exact problem as luismi. Standard recipe for granting sftp only access to a directory via "internal-sftp" in OpenSSH (sshd_config mods) lets the user access the directory, but with no write privileges. chmod of directory breaks the ability to login to the directory. (sftp seems happy with 755 on my box.)

As I understand the documentation for OpenBSD 4.3, chroot for sftp is independent of chroot for httpd (/var/www). So, I'd be surprised if adding files to ChrootDirectory grants the desired write privileges. The ability to break sftp by chmod-ing ChrootDirectory makes me think that sshd_config may require additional mods.

The novice (i.e. me) should be careful though, as these are Linux instructions.

I found that using "usermod -d" and "usermod -s" to change the user profile broke the login. However, creating an sftponly group works quite well. With a "Match Group sftponly" instruction in sshd_config, "usermod -g" easily adds a new user to the sftponly group.

Script wizards are encouraged to port the scripts presented in the Gentoo Forum listed above.